EXCEED THE SPACE PROVIDED. The prevalence of obesity in the United States has increased dramaticallyin the last decade. This increase comes in the face of public concerns over weight control and Federal guidelines to decrease the prevalence of obesity by the year 2000. This increase in the prevalence of obesity in such a short time provides strong evidence that obesity is not due solely to genetic factors, but that environmentalfactors also play a large role. Our previous studies indicates that one of the environmentalfactors is physical activity. In studies of previously obese women, we have demonstrated that individuals who are physicallyactive enough to raise their ratio of total energy expenditure to resting metabolic rate above 1.7 or 1.8 times resting metabolic rate gain little or no weight, whilehalf of those who are less active regain most of their lost weight. Others have shown that exercise increases fat oxidation in individuals consuming a high fat Western diet. We have preliminarydata that suggests that the mechanism involves increased oxidation of dietary fats consumed after exercise. Based on these finds, we propose three primary hypotheses: 1) Exercise increases the partitioningof subsequent dietary fat toward oxidation, 2) The mechanism of increased partitioning of fats toward oxidation is increased distribution of dietary fats to muscle tissue, and 3) Increased dietary fat oxidation will protect against weight gain. To test these hypotheses, we propose six human studies. The first of these validates a novel method of measuring fat oxidation using deuterated fatty acids. This method is superior to conventional 13C methods because it does not require confinement for measurement of carbon dioxide production. We will determine the influence of exercise intensity and time after exercise on the increased dietary fat oxidation. Muscle biopsy studies will be performed to measure muscle content of dietary fat using stable isotope tracers. A comparison of the effects of exercise on subsequent dietary fat oxidation will be made in lean, obese and previously obese individuals to test whether the effect is inherent in obese individuals or whether exercise is equally effect in all three groups. The final hypothesis will be tested by following weight regain in previously obese individualsto demonstrate if exercise induced dietary fat oxidation is protective against regain in active individuals. These studies will provide important guidelines for the prescription of exercise for the prevention and treatment of obesity.